BEIRUT — The Islamic State was routed Monday from one of its key strongholds on Syria’s border with Turkey after its defenses crumbled and its fighters either defected or fled, raising new questions about the group’s vaunted military capabilities. The fall of the town of Tal Abyad to a Kurdish-Syrian rebel force backed by U.S. airstrikes came after just two days of fighting during which the militants appeared to put up little resistance, focusing instead on escaping to their nearby self-styled capital of Raqqa or fleeing across the border to Turkey. The force — led by Kurdish units of the...